Natural gas is sometimes buried below the earth's surface in shale reservoirs. To recover this gas, the shale reservoirs are hydraulically fractured. Large volumes of frac fluid or gel are used to create a path for the gas to flow to the surface. Often shale reservoirs are associated with ground water. When the gas flows to the surface for recovery, the ground water and the water in the frac fluid flows to the surface as well. The recovered ground water and the water used to frac the shale reservoir, along with oil in some cases, are separated from the natural gas. Water separated from the natural gas is referred to as produced water.
The number of hydraulic fracturing shale gas wells in the United States and worldwide continues to increase. The United States has vast reservoirs of oil and natural gas which are commercially reachable as a result of hydraulic fracturing technologies. But as more hydraulic fracturing wells come into operation, so does the stress on surface water and ground water supplies from the withdrawal of large volumes of water used in the process. In some cases, for example, as much as one million gallons of fresh water per well head is required to complete the fracking process.
Equally important is the growing volume of wastewater generated from fracking wells which require disposal, recycling or treatment. In some cases, as much as 60% of the water injected into a well head during the fracking process will be discharged back out of the well shortly thereafter as frac flowback. Produced water is often contaminated with concentrations of constituents that require treatment before it can be reused or discharged to the environment. These contaminants typically include hydrocarbons from oil and inorganic salts. In particular, the contaminants in produced water typically include suspended solids, dissolved solids such as calcium, magnesium, sodium, barium, strontium, iron, chloride, etc.
There are three main approaches to dealing with wastewater in the oil and gas industry. The first, and by far the most common, is collecting the wastewater in large tanks and shipping it for disposal in underground injection wells. The second approach is treating the produced water, either onsite or at a centralized facility. This is particularly challenging since treatment standards are generally stringent and the cost is in some cases prohibitive. The third approach is to treat the produced water to lesser degrees and reuse it for fracturing fluid.
Considering the amount of water used or consumed in fracking operations and the need to conserve water resources, there is a need for produced water treatment processes that can meet stringent discharge requirement faced today and at the same time are cost effective.